...whenever you try to build outside the coordinate boundaries of 1000 units in any direction. You must be careful because though your brush coordinates may say 997 or -997 or something, that's just the CENTER of the brush, and if the brush is bigger than 6 units part of the brush may be beyond 1000.

Does this mean that 0-0-0 is the co-ordinate center and that all other constructions must be within a 2000-2000-2000 cube of that co-ordinate?

In other words that the maximum possible "Mission" size must be within the 1000 unit distance of 0-0-0.